System for stimulating bone growth, tissue healing and/or pain control, and method of use

ABSTRACT

A system for use in stimulating bone growth, tissue healing, and/or pain control is described. The system includes a screw, a battery, a controller, and means for connecting the battery such that current is routed from the battery through the screw and thence to a target area of interest requiring treatment. The screw includes an elongate shaft having a length extending between opposite ends. The shaft has an insulating coating extending along at least a portion of the length. The thickness of the insulating coating at various portions of the shaft is modulated to optimally direct current to a target area of interest requiring treatment. The controller adjusts the duty cycle of the current flow over the treatment period.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of U.S. patent applicationSer. No. 12/089,104 filed Aug. 30, 2008, which claims priority from U.S.Provisional Patent Application Ser. No. 60/813,633 filed on Oct. 3, 2005and PCT Patent Application No. PCT/US2006/038699 filed on Oct. 3, 2006,the contents of which are incorporated herein by reference in theirentirety.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

This invention was made with government support under Small BusinessInnovation Research Phase 1 Award No. 1248546 awarded by the NationalScience Foundation (NSF). The government has certain rights in theinvention.

BACKGROUND

The present invention relates generally to a system for stimulating bonegrowth and tissue healing, and more particularly to a method andapparatus for stimulating bone growth and tissue healing by applying anelectrical current to the bone and adjacent soft tissue through apartially insulated screw.

Bone growth is desirable in many instances, such as when vertebrae in apatient's spine are fused to overcome pain and other effects caused byinter-vertebral movement or intra-vertebral movement. Although bonegrowth occurs naturally, it can be stunted or stopped by various factorssuch as tobacco, alcohol and steroid usage, poor bone stock, and age.Moreover, stimulating bone growth to speed recovery is desirable in someinstances, such as when an injured athlete wishes to return to her sportquickly. Other motivations for stimulating bone growth are to reducechronic pain, to improve mobility, and avoid future complications. Thus,there is a need for stimulating bone growth in individuals.

Bone growth, tissue healing and pain control can be stimulated byvarious means. One such means for stimulating bone growth, tissuehealing and pain control is by passing an electrical current through thebone. As one example, when fusing vertebrae in a patient's spine,various means have been used to stimulate bone growth. For example, somestimulators include wire electrodes embedded in bone fragments graftedto a region of the patient's back containing the vertebrae to be fused.Direct or alternating electrical current is applied to the electrodes tostimulate bone growth and fuse the fragments and adjoining vertebrae. Topermit the current to be applied for extended periods of time whilepermitting the patient to be mobile, a generator is connected to thewire electrodes and implanted between the skin and muscle near thepatient's vertebral column. The generator provides a continuous lowamperage direct current (e.g., 20-100 μA) for an extended period of time(e.g., six or more months). After the vertebrae are fused, the generatorand leads are surgically removed. Although these embedded electrodes aregenerally effective, the wire electrodes are susceptible to failure,requiring additional surgery to repair them. Moreover, placement of thewire electrodes is less than precise, allowing some of the current topass through areas of tissue and bone where it is unneeded and where thecurrent could potentially have adverse effects. Further, due toimprecise placement or lack of proximity to an area of interest, moreenergy must be provided to the electrodes than otherwise is necessary tobe optimally effective. Thus, there are several drawbacks and potentialproblems associated with devices such as these.

Although small amounts of mechanical loading can stimulate growth, it isgenerally desirable to limit movement between the bones or bonefragments being fused. There are several known means for limiting bonemovement. Among these means for limiting bone movement are plates, rodsand screws. The plates and rods are typically held in position by screwswhich are mounted in the bone or bones being fused. FIG. 1 illustratesscrews (generally designated by 10) driven into a vertebra 12 toimmobilize the vertebra. As previously mentioned, the screws 10 are usedfor attaching rods 14 and/or plates (not shown) to vertebrae to hold thevertebrae in position while they fuse. Although these screws 10 workwell for their intended purpose, they only provide mechanical fixation,and do not provide other potential benefits, such as facilitatingelectrical stimulation of the region and lack of adapting with changingtissue environments. In addition, with such conventional screws,undesirable complications may include loosening over time; being proneto pullout; being prone to infection; and not being useful in degradedosteoporotic or compromised bone. Moreover, if electrical stimulationwere applied to bones using conventional screws, the screws 10 would notfocus therapeutic stimulation and bone growth to anatomical areas whereit is most desired and/or needed. Rather, they could potentially conductcurrent to areas of tissue and bone where the current is unneeded andwhere the current could potentially have adverse effects. Thus, thereare drawbacks and potential problems associated with conventional screwssuch as these.

Beyond the well-defined role of electrical fields within bone formation,electrical fields have also shown significant promise in aiding healingand recovery in nerve and spinal cord injury. Stimulating tissue healingwith electrical currents has been demonstrated to be efficacious inanimal models and is now being attempted experimentally in humansubjects. Further, spinal cord and nerve root injury has been known tocause associated debilitating pain syndromes which are resistant totreatment. These pain syndromes also have shown improvement with pulsedelectrical stimulation. Given these findings, it is envisioned that asystem and/or an apparatus providing a specified and confined electricalfield through bony constructs and adjacent tissue (e.g., neural tissue)will facilitate an enhanced recovery from spinal cord and nerve injury,including improved functional outcome, better wound healing, and ahigher level of pain control.

In U.S. Pat. No. 3,918,440, Kraus teaches the use of alternating current(A.C.) for assisting in the healing of bone damage. A.C. current carriesseveral disadvantages. A.C. current relies on a complex power supply. Inaddition, it is more difficult to predict and control the spatialdistribution of A.C. current within a body, since current may flow boththrough resistive and capacitive paths. Overall, substitution of D.C.for A.C. current results in system simplifications and opportunity toimprove precision in targeting treatment to particular areas of interestwithin the body, while avoiding collateral damage to surroundingtissues. D.C. current is potentially advantageous in that requiredenergy can be provided by batteries. However, it is critically importantto properly size the battery powering a D.C. stimulation system toprevent premature interruption of the scheduled treatment. In fact,engineering tradeoffs include at least battery size, voltage, amp-hours,self-discharge rate, cost, and form factor. Clearly, there is a need fora D.C. stimulation system that optimally conserves power and allows forstimulation of bone growth and tissue healing. A smaller, lower costbattery will lead to increased patient mobility and comfort.

SUMMARY

In one aspect, a system for stimulating at least one of bone growth,tissue healing, and pain control is provided, wherein the system exertsboth spatial and temporal control over D.C. current flow. The systemcomprises a D.C. battery and means for connecting the positive terminalof the battery to a screw that is inserted into a portion of a bodyrequiring bone growth, tissue healing, or pain control, therebydirecting current to a targeted area. In addition, the system comprisesmeans to direct current exiting the body to the negative terminal of thebattery, and controller means to controllably adjust the duty cycle ofthe current according to a prescribed schedule.

In one aspect, a screw for use in stimulating at least one of bonegrowth, tissue healing, and pain control is provided. The screwgenerally comprises an elongate shaft having a length extending betweenopposite ends, an exterior surface, and a screw thread formed on theexterior surface of the shaft and extending along at least a portion ofthe length of the shaft. The shaft also has an insulating coatingextending along at least a portion of the length. The screw furthercomprises a head adjacent one end of the shaft for engaging the screw torotate the screw and thereby drive it into bone, and an electricalconductor electrically connectable to the shaft for conveying currentthrough the shaft. A thickness of the insulating coating at a firstportion of the shaft is greater than a thickness of the insulatingcoating at a second portion of the shaft.

In another aspect, an apparatus for stimulating at least one of bonegrowth, tissue healing, and pain control is provided. The apparatusgenerally comprises an electrical power source and a plurality ofelectrodes. The plurality of electrodes are electrically connected tothe electrical power source with at least one of the electrodes having atip adapted for screwing into a patient and an insulating coatingextending along at least a portion of its length. The insulating coatingat a first portion of electrode is sufficiently thick to substantiallyprevent flow of current, while the insulating coating at a secondportion of the electrode is sufficiently thin to substantially allowflow of current.

In yet another aspect, a method of stimulating at least one of bonegrowth, tissue healing and pain control is provided. The methodgenerally comprises inserting a first and second electrode into apatient with the second electrode inserted at a predetermined distancefrom the first electrode, and applying an electric current to at leastone of the first electrode and the second electrode. A thickness of aninsulating coating at a second portion of the first electrode is greaterthan a thickness of the insulating coating at the first portion of thefirst electrode, and a thickness of an insulating coating at a secondportion of the second electrode is greater than a thickness of theinsulating coating at the first portion of the second electrode suchthat the electric current passes through the patient between the firstportion of the first electrode and the first portion of the secondelectrode.

In another aspect, a method of producing an electrode is provided. Themethod generally comprises the formation of an electrode where thethickness of an insulating coating at a second portion of the electrodeis greater than a thickness of the insulating coating at the firstportion of the electrode. The method comprises the controlled immersionof a metallic electrode into a bath, application of an electrical chargeor current, and controlled extraction of the metallic electrode from thebath. The depth of immersion, orientation of immersion, time ofimmersion, rate of immersion, composition of the bath, polarity of theelectric charge, amplitude of the electric charge, rate of extraction,and distance of extraction may be controlled in order to achievespecific thicknesses of insulating coatings on the electrode surface.

In another embodiment, a portion of the length of the electrode may beuniformly coated such that the thickness of the insulating coating isthe same over the entire length of the insulating coating. The relativelength of the coated region of the electrode may therefore be variedfrom 50% to 95% of the length of the electrode. In this particularembodiment a method may be employed to achieve a uniform coating over aportion of the length of the electrode. The method generally comprisesthe rapid immersion of the metallic electrode into a bath along apredetermined orientation of immersion and up to a predetermined depthof immersion. The method further comprises a predetermined polarity ofthe electric charge, amplitude of the electric charge, and time ofimmersion. Finally, the method comprises the rapid and completeextraction of the metallic electrode from the bath.

In another embodiment, the coated portion of the electrode comprises100% of the length of the electrode and the thickness of the coating isgreatest at one end of the electrode and decreases along the entirelength of the electrode. For example, in one embodiment the thickness ofthe coating is approximately 400 nanometers at the end of the electrodeand decreases in thickness until it reaches approximately zero near theopposite end of the electrode. According to this embodiment, theinsulating thickness can be graded, for example, linearly orexponentially. Again, the specific dimensions are merely illustrativeand in other embodiments the thickness of the coating along the lengthof the electrode may vary from those depicted without departing from thescope of this disclosure. In this particular embodiment a method may beemployed to achieve a graded coating over substantially the entirelength of the electrode. The method generally comprises the rapidimmersion of the metallic electrode into a bath along a predeterminedorientation of immersion and up to a predetermined depth of immersion.The method further comprises a predetermined polarity of the electriccharge, amplitude of the electric charge, and time of immersion.Finally, the method comprises the slow and controlled extraction of themetallic electrode from the bath along a pre-determined andvariable-speed course of extraction. Specifically, extraction of theelectrode from the bath at a constant rate will produce a lineargradient in the thickness of the coating from one end of the electrodeto the other end of the electrode. Furthermore, extraction of theelectrode from the bath at a progressively decreasing rate will producean exponential gradient in the thickness of the coating from one end ofthe electrode to the other end of the electrode. As will be apparent tothose skilled in the arts, any arbitrary gradient of insulating coatingcan be produced by appropriate adjustment of the rate of extraction ofthe electrode from the bath.

In another embodiment, the coated portion of the electrode comprisesbetween 50% and 95% of the length of the electrode and the thickness ofthe coating is greatest at one end of the electrode and decreases alongthe length of the electrode. For example, in one embodiment thethickness of the coating is approximately 400 nanometers along a pre-setlength of the electrode and then decreases in thickness, for examplelinearly or exponentially, until it reaches approximately zero near theopposite end of the electrode. The method generally comprises the rapidimmersion of the metallic electrode into a bath along a predeterminedorientation of immersion and up to a predetermined depth of immersion.The method further comprises a predetermined polarity of the electriccharge, amplitude of the electric charge, and time of immersion.Finally, the method comprises the slow and controlled immersion of themetallic electrode from the bath along a pre-determined and variablespeed course of extraction. Specifically, immersion of the electrodeinto the bath at a constant rate will produce a linear gradient in thethickness of the coating from the coated portion of the electrode to theend of the electrode. Furthermore, immersion of the electrode into thebath at a progressively increasing rate will produce an exponentialgradient in the thickness of the coating from one end of the electrodeto the other end of the electrode.

In another embodiment, the coated portion of the electrode comprisesbetween 5% and 95% of the length of the electrode and the thickness ofthe coating is approximately zero at given portions of the length andvaries according to a predetermined schedule at other portions of thelength of the electrode. For example, in one embodiment a protectivecoating such as wax or photoresist is applied at portions of the lengthwhere it is desired to have approximately zero thickness of the coating,and the thickness of the coating varies from approximately 400nanometers along a pre-set length of the electrode to approximately zeronear the opposite end of the electrode. The method generally comprisesthe rapid immersion of the metallic electrode into a bath along apredetermined orientation of immersion and up to a predetermined depthof immersion. The method further comprises a predetermined polarity ofthe electric charge, amplitude of the electric charge, and time ofimmersion. Finally, the method comprises the slow and controlledimmersion of the metallic electrode from the bath along a pre-determinedand variable speed course of extraction. Specifically, immersion of theelectrode into the bath at a constant rate will produce a lineargradient in the thickness of the coating from the coated portion of theelectrode to the end of the electrode. Furthermore, immersion of theelectrode into the bath at a progressively increasing rate will producean exponential gradient in the thickness of the coating from one end ofthe electrode to the other end of the electrode.

Other features of the present invention will be in part apparent and inpart pointed out hereinafter.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a horizontal cross section of a conventional electricallyconductive screw installed in a vertebra;

FIG. 2 is a side elevation of a screw of the present invention;

FIG. 3 is a cross section of the screw taken along line 3-3 of FIG. 2;

FIG. 4 is an front elevation of a portion of a spine with a firstapparatus of the present invention including a plate installed thereon;

FIG. 5 is a side elevation of a portion of a spine with a secondapparatus of the present invention installed thereon;

FIG. 6 is a schematic of electric fields induced by an unanodized screwfor various levels of an applied direct current;

FIG. 7 is a schematic of an electric field induced by the unanodizedscrew and electric fields induced by three embodiments with anodizationlayers covering 50%, 90%, and 95% of the length of the screw;

FIG. 8 is a plot depicting a thickness of an anodization layer on thescrew for four different anodization gradients according to aspects ofthe disclosure; and

FIG. 9 is a schematic of an electric field induced by the screwemploying each of the anodization gradients depicted in FIG. 8.

FIG. 10 is a schematic of a system comprised of a battery, a controllerfor adjusting application of D.C. current over time, a screw functioningas a first electrode, and a ground electrode.

FIG. 11 is a cross section of a portion of a screw in which thethickness of an insulating coating is varied.

Corresponding reference characters indicate corresponding partsthroughout the several views of the drawings.

DETAILED DESCRIPTION OF AN EMBODIMENT

Referring now to the drawings and in particular to FIG. 2, a screw orelectrode of the present invention is designated in its entirety by thereference numeral 20. The screw 20 has an elongate shaft 22 having alength 24 (FIG. 3) extending between opposite ends 26, 28. Aconventional screw thread 30 is formed on an exterior surface 32 of theshaft 22. The thread 30 extends along at least a portion of the length24 of the shaft 22. The screw 20 also includes a head 34 adjacent theone end 28 of the shaft 22. The head 34 is shaped for engaging the screw20 with a driver or wrench to rotate the screw and thereby drive it intobone. In one embodiment, the screw 20 includes a connector, generallydesignated by 36, adjacent its head 34 for connecting an electricalconductor to the screw as will be explained in further detail below. Inother embodiments, electrical current is coupled to the screw 20 throughany other suitable coupling, such as through a rod or tulip. In general,the electrical connection is made to a portion of an assembly includingthe screw 20, where the assembly is fully insulated with the exceptionof the region where current is desired. In one embodiment, the connector36 includes a screw fastener 38 threaded into the screw 20 for holdingthe electrical lead. As illustrated in FIG. 2, an electrical conductor40 is electrically connectable to the screw 20 and to an electricalpower source 42 for conveying electrical current through the shaft 22.In one embodiment the power source 42 produces direct current. Inanother embodiment, the power source 42 produces alternating currentsuch as a time-varying current waveform (e.g., a sine wave or a squarewave) having a frequency between nearly zero hertz and ten gigahertz. Inyet another embodiment, the power source 42 provides a direct current tothe screw 20 and/or provide a pulsed direct current to the screw via oneor more waveforms such that periods of stimulation (i.e., periods ofcurrent being delivered to the screw) are intermixed with periods ofrecovery (i.e. periods where a reduced current or even no current isbeing delivered to the screw) as will be discussed more fully. Althoughother electrical conductors 40 may be used without departing from thescope of the present invention, in one embodiment the conductor is a 35gauge insulated braided stainless steel wire. In other embodiments, theelectrical conductor 40 may be omitted altogether, such as embodimentswhere the screw 20 receives an electrical current wirelessly or when thepower source 42 is integral to the screw, as will be discussed morefully. It is further envisioned that the connector 36 may take otherforms, for example but not limited to a rod or tulip. For example, theconnector 36 may be a threaded terminal and nut, a fastenerlessconnector, a quick disconnect type connector, a soldered pin, or anadhesive without departing from the scope of the present invention.Further, and although in the depicted embodiment the conductor 40 isfixed generally perpendicular to an center axis of the screw 20extending in a direction of the screw's length 24, in other embodimentsthe connector 36 may connect the conductor at any suitable anglerelative to the center axis of the screw without departing from thescope of this disclosure. It will be apparent to those skilled in thearts that ground electrode may be separated into multiple componentsthat are spatially separated.

According to an embodiment where screw 20 receives current wirelessly, acontroller included in a first external circuit controls when power istransmitted to a second circuit formed by screw 20 and ground electrode.In this embodiment, said second circuit amounts to the secondary of anair-core transformer.

According to yet another embodiment, screw 20 comprises a screw bodyhaving a cavity, wherein power source 42 comprises a battery included insaid cavity. A first terminal of power source 42 is attached directly toscrew 20, while a second terminal of power source 42 is attached to aground electrode.

As illustrated in FIG. 3, the shaft 22 is generally conductive, but aportion of the shaft is coated with an insulating coating 50. Thus, theshaft 22 has an electrically conducting portion 52 and an electricallyinsulating portion 54. Although the conducting portion 52 of the screw20 may have other lengths without departing from the scope of thepresent invention, in one embodiment the conducting portion of the screwhas a length of less than about four centimeters. In one embodiment, theconducting portion 52 of the screw 20 has a length of between aboutthree millimeters and about three centimeters. Further, although theconducting portion 52 of the screw 20 may be positioned at otherlocations along the screw, in one embodiment the conducting portion ofthe screw is positioned adjacent the end 26 of the screw opposite thehead 34. In another embodiment (not shown), the conducting portion 52 ofthe screw 20 is positioned between the ends 26, 28 of the screw, andeach end of the screw is electrically insulated. Although the insulatingportion 54 of the screw 20 may have other lengths without departing fromthe scope of the present invention, in one embodiment the insulatingportion of the shaft 22 extends at least forty percent of the length 24of the screw. In another embodiment, the insulating portion 54 of theshaft 22 extends between about fifty percent of the length 24 of thescrew 20 and about ninety five percent of the length of the screw.

In one embodiment, a clevis 60, sometimes referred to as a tulip, isattached to the screw 20. The clevis 60 pivots freely on the head 34 ofthe screw 20 and includes a pair of legs 62 defining an opening 64adapted to receive a rod 66. The legs 62 include threads 68 for engaginga screw 20 for fastening the rod 66 in the opening 64 and preventing theclevis 60 from pivoting on the screw head 34. Other features of thescrew 20 and clevis 60 are conventional and will not be described infurther detail.

As will be appreciated by those skilled in the art, the screw 20comprises an electrically conductive material such as a titanium alloyand the electrically insulating portion of the shaft is coated with aninsulating material 50 such as titanium dioxide. In one embodiment, theinsulating material 50 is formed by anodizing the exterior surface 32 ofa portion of the shaft 22, including the head 34. In some embodiments,the insulating material 50 is an anodization layer with a variablethickness (i.e., a gradient), as will be discussed more fully. Theconductivity of the screw 20 in the conducting portion 52 may beimproved by coating the screw with a highly conductive materialincluding but not limited to titanium nitride, platinum, or an alloy ofplatinum and iridium. Both treated surfaces, titanium dioxide andconductive material, are extremely adherent to the titanium andtherefore not likely to be breached when screwed into bone. Becausemethods for anodizing and/or coating titanium parts are well known bythose having ordinary skill in the art, they will not be described infurther detail.

The screw 20 is used in conjunction with a ground electrode (not shown)so that an electrical circuit is completed from electrical power source42 into the bone or tissue into which the screw are driven, and thenceto said ground electrode, which optionally may be a second screw. Aswill be appreciated by those skilled in the art, electrical currenttravels through the conductive portion 52 of the screw 20 fromelectrical power source 42 and/or conductor 40 to the bone in which thescrew 20 is inserted (e.g., a vertebra such as vertebra 12 in FIG. 1) aswill be explained in more detail below. The current does not passthrough the coating 50 (e.g., anodization layer) on the insulatedportion 54 of the shaft 22 so that the current may be directed to theportion of the bone or other tissue where stimulation is most needed. Aswill be also appreciated, the insulated portion 54 of the shaft 22reduces current from passing through portions of the bone and tissuewhere electrical current is not desired. The screw 20 of the presentinvention may be installed in the bone using conventional techniques. Inmost instances, the bone is pre-drilled to avoid splitting when thescrew 20 is installed. It is envisioned in some instances the bone maybe reinforced, such as with bands before the screw 20 is installed toprovide support to the bone and prevent damage to it as the screw isinstalled.

In some instances, it is envisioned that the screws 20 of the presentinvention may be used in combination with other appliances, such asspacers, BMP sponges, synthetic bone substitutes, IV discs, cages, etc.For example, in some applications the screws 20 may be installed througha plate 80 as shown in FIG. 4 to provide support for the bone and toguide proper spacing and positioning of the screws. In this embodiment,plate 80 has at least two openings (not shown) for receiving screws 20.Preferably, each of the openings are sized and shaped for receiving atleast one screw 20. Although the openings in the plate 80 may have otherspacing without departing from the scope of the present invention, inone embodiment the openings are spaced by a distance 82 of between aboutone centimeter and about two centimeters. In the embodiment shown inFIG. 5, the spacers are formed as rods 66 bridging the screws 20 asdescribed above. Rods 66 and plate 80 are electrically conductive, butcompletely anodized. As the configurations shown in FIGS. 4 and 5 areotherwise known to those having ordinary skill in the art, they will notbe described in further detail.

To use the apparatus of the present invention to stimulate bone growth,the bone (e.g., vertebra 12) is pre-drilled. A first screw 20 isinserted in the bone and driven into place by turning the screw. Asecond screw 20 is inserted in the bone at a predetermined distance fromthe first screw. In other embodiments, only first screw 20 is needed andsecond crew 20 is omitted. Next, an electrical connection is made, suchas by attachment of conductors 40 to screws 20, rods 66, and/or clevis60, between the screws 20 and to an electrical power source 42 (e.g., agenerator, a battery or an inductance coil positioned in a pulsingmagnetic field). The conductors 40 are energized by the power source 42so an electrical current passes through the bone. As will be discussedin more detail, in some embodiments the conductor 40 may optionally beomitted, such as, e.g., in embodiments where the power source 42 isintegral to one or more screws 20 and/or when one or more screws receivean electrical current wirelessly from the power source 42. Further, insome embodiments the conductor 40 may be attached to other componentssuch as, e.g., the rods 66, which in turn may conduct the receivedcurrent to the screws 20 as will be discussed more fully. Because thescrews 20 are partially insulated, the electrical current passes betweenonly a portion of the first screw and only a portion of the groundelectrode directing the current to a particular area of the bone ortissue. Although other amounts of current may be used, in one embodimenta direct current of between about one microamp and about one milliamp isused. In another embodiment, a direct current of between about twentymicroamps and about sixty microamps is used. In other embodiments, adirect current of about twenty, forty, sixty, eighty, or one hundredmicroamps is used. In other embodiments, the current may be anytime-varying current waveform (e.g., a sine wave or a square wave)having a frequency between nearly zero hertz and ten gigahertz. In stillother embodiments, a current may be pulsed, provided according to a dutycycle, and/or provided according to one or more waveforms such as adirect current sine wave or a direct current square wave as will bediscussed more fully.

In addition to stimulating bone growth, it is envisioned that theapparatus and method described above may be used to improve tissuegrowth and healing, including soft tissue and nerve tissue. Thus, theapparatus and method may be useful in healing spinal cord and nerve rootinjury. Further, in some embodiments, the apparatus and method may beuseful in treating pain syndromes.

As discussed, in some embodiments the electrically conducting portion 52of the screw 20 may extend for less than the entire length 24 of thescrew. In such embodiments, an electric field induced in an environmentwhere the screw 20 is implanted (e.g., a bone, tissue, etc.) may bealtered and/or focused on an area of interest (i.e., an area of thebone/tissue requiring stimulation). This may be more readily understoodwith reference to FIG. 6. FIG. 6 illustrates an electric field inducedby various amplitudes of an applied direct current for a screw 20containing no insulating coating 50 (i.e., a screw with the electricallyconducting portion 52 extending for the entire length 24 of the screw).More specifically, FIG. 6 illustrates an electric field induced by ascrew 20 without the insulating coating 50 for a direct currentstimulation of 20 microamps in FIG. 6A, 40 microamps in FIG. 6B, 60microamps in FIG. 6C, 80 microamps in FIG. 6D, and 100 microamps in FIG.6E. As depicted, when the screw 20 does not contain the insulatingcoating 50, the electric field induced in the bone and/or surroundingtissue where the screw is implanted is generally elliptical in shape andextends over the entire length 24 of the screw. Further, as theamplitude of the current applied to the screw 20 is increased, theamplitude of the induced electric field is generally increased as well.Thus, for a screw 20 containing no insulating coating 50, the electricfield is not focused at any one portion along the length of the screw20, but rather extends along the entire length of the screw. Further, ifthe amplitude of the induced electric field at any one point along thescrew 20 needs to be increased (in order to, e.g., achieve appropriateosteogenic benefits) the amplitude along the entire length of the screwmust be increased. This may result in electric fields being induced inareas not of interest (e.g., electric fields may be induced near the end28 of the screw 20 where stimulation may not be needed). This may alsoresult in large amounts of power consumed as the current applied to thescrew may need to be dramatically increased in order to produce adesired amplitude of the induced electric field at the any of interest(i.e., the area of the bone and/or surrounding tissue requiringosteogenic stimulation).

However, when the insulating coating 50 is applied to the screw 20 (suchas by anodization or the like as discussed) forming the electricallyinsulating portion 54, the geometry of the induced electric field may bealtered and the induced electric field may be more readily focused onthe area of interest. In some embodiments, an anodization layer isapplied to the screw over less than the entire length 24 of the screwforming the insulating coating 50. The thickness of the insulatingcoating 50 is directly related to the degree of insulation, currentejection, and resistance. In some embodiments, the insulating coating 50has a substantially uniform thickness. In other embodiments, thethickness of insulating coating 50 is varied (as depicted in FIG. 11).This anodization layer (i.e., insulating coating) alters the geometryand intensity of the induced electric field at the area of interest. Forexample, a portion of the length 24 of the screw 20 may be uniformlyanodized such that a thickness of the anodization layer is the same overthe entire length of the insulating coating 50. In some embodiments, arelative length of the insulating coating 50 (i.e., the anodized region)of the screw 20 to the overall length 24 of the screw may be about 50%.In other embodiments, the length of the anodized region of the screw 20relative to the overall length 24 of the screw may be about 75%, 90%, oreven 95%.

In such embodiments, the geometry and amplitude of the induced electricfield in the environment surrounding the screw 20 differs from thegeometry and amplitude of the electric field induced by a screwcontaining no insulating coating 50 (as discussed in connection withFIG. 6). This may be more readily understood with reference to FIG. 7.FIG. 7 illustrates an electric field induced by a screw 20 comprisingvarious lengths of the insulating coating 50 (i.e., the anodized region)for a constant supplied direct current. Specifically, FIG. 7Aillustrates an electric field induced by a screw 20 having no portion ofits length 24 anodized (similar to the screw discussed in connectionwith FIG. 6), FIG. 7B illustrates an electric field induced by a screwhaving 50% of its length anodized, FIG. 7C illustrates an electric fieldinduced by a screw having 90% of its length anodized, and FIG. 7Dillustrates an electric field induced by a screw having 95% of itslength anodized. As discussed, for a screw 20 having no insulatingcoating 50, the induced electric field (as depicted in FIG. 7A) isapproximately elliptical in shape and extends over the entire length 24of the screw. However, for a screw 20 having an anodization layer (i.e.,insulating coating 50), the induced electric field (as depicted in FIGS.7B-7D for relative coating lengths of 50%, 90%, and 95%, respectively)is approximately spherical in shape. More particularly, when the screw20 comprises the insulating coating 50, a spherical electric field isinduced which is centered around the electrically conducting portion 52of the screw.

Further, the amplitude of the induced electric field centered around theelectrically conducting portion 52 of the screw 20 increases as therelative length of the insulating coating 50 increases for a constantdirect current applied to the screw, which may increase the therapeuticeffect. For example, for a given direct current (e.g., 40 microamps),the electric field induced near the end 26 of the screw 20 having 95% ofits length anodized extends further into the bone and/or surroundingtissue and has a higher intensity than in embodiments where the screwhas less than 95% of its length anodized. Moreover, the spatialdistribution of the induced electric field extends farther from thesurface of the screw. Accordingly, in some embodiments the length 24 ofthe screw 20 may be selectively anodized to control the induced electricfield geometry and amplitude. Specifically, a portion of the length 24of the screw 20 may be anodized in order to achieve a generallyspherical electric field, and an appropriate percentage of the length ofthe screw may be anodized in order to concentrate the electric field onan area of interest (i.e., the volume of the bone and/or surroundingtissue requiring osteogenic stimulation). As a result, areas of interestwithin the bone and/or its surrounding tissue may be suitably stimulatedwhile using less power and/or a decreased period of stimulation than isneeded for screws 20 containing no insulating coating 50.

In some embodiments, a thickness of the insulating coating 50 (e.g., athickness of the anodization layer) may vary over the portion of thescrew 20 containing the insulating coating, as depicted in FIG. 11, inorder to produce a gradient of the insulating coating. For example, thethickness of the anodization layer may be greater at one point along thelength 24 of the screw 20 than at another point, providing furtherosteogenic stimulation benefits. This may be more readily understoodwith reference to FIGS. 8 and 9. First, FIG. 8 depicts four plotsillustrating a thickness of an anodization layer (i.e., insulatingcoating 50) over the length 24 of the screw 20 for four illustrativeembodiments of the disclosure. More particularly, FIG. 8 depicts plotsdepicting the thickness of the anodization layer over the length 24 ofthe screw 20 for a 100% exponential gradient 90, a 100% linear gradient92, a 50% exponential gradient 94, and a 50% linear gradient 96.Although FIG. 8 depicts specific thickness dimensions of the anodizationlayer for each embodiment (ranging from 400 nanometers to zeronanometers), it should be appreciated that in other embodiments theanodization layer may be any suitable thickness without departing fromthe scope of this disclosure. Further, while FIG. 8 only depictsgradients of the anodization layer covering either 100% of the length 24of the screw 20 or 50% of the length of the screw, it should beappreciated that in other embodiments the gradient may cover anysuitable percentage of the length of the screw without departing fromthe scope of this disclosure. In various embodiments, the percentage ofthe length of the screw 20 which is anodized can be any percentage andin any pattern (e.g., exponential, logarithmic, linear, etc.) suitablefor a particular or general clinical application.

Returning to FIG. 8, in embodiments where the screw 20 comprises the100% exponential gradient 90, the thickness of the anodization layerwill be the greatest at one end of the screw 20 (i.e., either end 28 orend 26) and will decrease exponentially along the entire length of thescrew 20. For example, in the depicted embodiment a thickness of theanodization layer is approximately 400 nanometers at the end 28 of thescrew 20 and decreases in thickness exponentially until it reachesapproximately zero near the opposite end 26 of the screw. Again, thespecific dimensions depicted in FIG. 8 are merely illustrative and inother embodiments the thickness of the anodization layer along thelength 24 of the screw 20 may vary from those depicted without departingfrom the scope of this disclosure. In embodiments where the screw 20comprises the 100% linear gradient 92, the thickness of the anodizationlayer will be the greatest at one end of the screw 20 (i.e., either end28 or end 26) and will decrease linearly (e.g., decrease at a constantrate) along the entire length 24 of the screw. For example, in depictedembodiment a thickness of the anodization layer is approximately 400nanometers at the end 28 of the screw 20 and decreases in thicknesslinearly until it reaches approximately zero near the opposite end 26 ofthe screw. For the 50% exponential gradient 94 and the 50% lineargradient 96, the thickness of the anodization layer follows a similarpattern as that of the 100% exponential gradient 90 and the 100% lineargradient 92, respectively, except that each of the gradients 94, 96maintain a constant thickness of the anodization layer overapproximately 50% of the length 24 of the screw 20 (in the depictedembodiment, 400 nanometers), and then decreases in thickness along therest of the length either exponentially (for the 50% exponentialgradient) or linearly (for the 50% linear gradient).

When the screw 20 comprises a graded anodization layer as describedabove, the geometry and amplitude of a resulting induced electric fieldmay differ from those induced in embodiments where the screw comprises auniform anodization layer thickness (as discussed in connection withFIG. 7) and in embodiments where the screw comprises no anodizationlayer (as discussed in connection with FIG. 6). This may be more readilyunderstood with reference to FIG. 9. FIG. 9 depicts an induced electricfield for the four anodization gradients depicted in FIG. 8 under aconstant direct current. Specifically, FIG. 9A depicts an inducedelectric field for the 100% linear gradient 92, FIG. 9B depicts aninduced electric field for the 50% linear gradient 96, FIG. 9C depictsan induced electric field for the 100% exponential gradient 90, and FIG.9D depicts an induced electric field for the 50% exponential gradient94. As seen, when the screw 20 comprises an insulating coating 50 with avarying thickness (e.g., a graded anodization layer as depicted in FIG.11) the resulting induced electric field is generally pear-shaped (i.e.,it includes a relatively narrow elongated portion along a portion of theshaft of the screw 20 that relatively rapidly expands to broader,rounder shape). More particularly, the induced electric field near adistal end 26 of the screw 20 is higher in amplitude and extends over alarger spatial region, while the induced electric field near theproximal end 28 of the screw is lower in amplitude and extends only asmall distance from the screw exterior surface 32. Further, forgradients where the thickness of the anodization layer over half of thelength 24 of the screw 20 was kept constant (e.g., the 50% exponentialgradient 94 and the 50% linear gradient 96), the amplitude of theinduced electric field is also pear shaped yet the electric field nearthe distal end 26 is even higher in intensity and extends further fromthe exterior surface 32 of screw, and the electric field near theproximal end 28 is even lower in amplitude and extends a smallerdistance from the exterior surface of the screw.

Thus, in some embodiments the screw 20 may be selectively anodized witha graded anodization layer in order to control the induced electricfield geometry and amplitude. Specifically, a portion of the length 24of the screw 20 may be anodized with at least part of the anodizedportion having a varying thickness of the anodization layer in order toachieve a generally pear-shaped electric field. As a result, areas ofinterest within the bone and/or its surrounding tissue may be suitablystimulated while using less power and/or a decreased period ofstimulation than is needed for screws 20 containing no insulatingcoating 50.

In some embodiments, the screw 20 may be selectively anodized in orderto achieve desired properties for a particular clinical setting. Thatis, depending on the specific bone, tissue, etc., ultimately stimulatedby the screw 20, one or more of the above discussed anodization layerpatterns may be applied such that an electric field induced by the screwdelivers an appropriate electric field to a stimulated area of interest.

For example, and as will be appreciated by those having skill in the artgiven the benefit of this disclosure, in some embodiments the screw 20may be implanted in or near human vertebrae (as depicted in FIGS. 4 and5) to provide osteogenic stimulation to the vertebrae and/or thesurrounding tissue. In such embodiments, the anodization pattern of thescrew 20 may be configured according to a specific region requiringstimulation. For example, in embodiments where stimulation is desired inthe intervertebral (IV) space or a vertebral body, an uniformanodization pattern with 95% or more of the length 24 of the screw 20anodized may provide the greatest osteogenic benefits. However, inembodiments where stimulation is desired in an instrumented pedicle, anexponentially or linearly graded anodization pattern (as compared to auniform anodization pattern) may provide the greatest osteogenicbenefits. Further, in embodiments where stimulation is desired in eachof the IV space, the vertebral body, and the instrumented pedicle, alinearly graded anodization layer (such as a 100% linearly gradedanodization layer) may provide the greatest osteogenic benefits.Accordingly, a length of the anodization layer on the screw 20 and/or agradient of the anodization layer on the screw may be configuredaccording to a desired osteogenic application of the screw.

In some embodiments, selective anodization (using any of the anodizationpatterns as discussed) may be provided anywhere along the length 24 ofthe screw 20, as depicted in FIG. 11, without departing from the scopeof this disclosure. For example, in some embodiments the screw 20 maycomprise the conducting portion 52 of the screw at a different relativelocation than that depicted in, e.g., FIG. 3. More particularly, in someembodiments the conducting portion 52 of the screw 20 may be positionedbetween the ends 26, 28 of the screw, with each end of the screw beingelectrically insulated. In such embodiments, an anodization layer may beprovided to each of the ends 26, 28 of the screw forming the insulatingcoating 50, with a portion of the screw between the anodization layersleft exposed to form the conducting portion 52. The anodization layersprovided to either end may employ any of the above discussed anodizationpatterns. For example, in some embodiments, a thickness of theanodization layers at the ends 26, 28 of the screw 20 may be greaterthan a thickness of the anodization layer nearer a midpoint of the screw20 such that that one or more of the above discussed benefits of thegraded anodization layer is provided at a different relative locationalong the screw. Generally, the length and position of theunanodized/uncoated region of the screw 20 may be varied depending onthe configuration and the specific clinical application.

In some embodiments, the electrical conductor 40 may be omitted withoutdeparting from the scope of the disclosure. For example, in someembodiments, the electrical power source 42 may be integral to the screw20, such as in the form of a battery. In other embodiments, theelectrical power source 42 may be external to the screw 20 but maynonetheless not be connected to the screw. For example, the electricalpower source 42 may conduct electricity to the screw 20 via one or morewell-known wireless power delivery methods.

Further, and because an intensity and/or relative spatial distance ofthe induced electric field of the screw 20 may be increased for a givenapplied DC current as compared to known electric stimulators asdiscussed, less current may ultimately be needed to achieve a desiredelectric field intensity and associated osteogenic stimulation. Forexample, in embodiments where the screw 20 is anodized over 95% of itslength, induced electric fields within the IV space and vertebral bodymay be over 500% greater in amplitude than those induced by unanodizedscrews (i.e., screws containing no insulating coating 50). Accordingly,this may lead to increased battery life and/or reduced power consumptionas compared to known electric stimulators. In such embodiments, aninternal battery and/or wireless power delivery may be used even if suchdelivery methods were previously impractical due to the relatively highcurrent needed for unanodized screws 20 as discussed.

In other embodiments, the electrical conductor 40 may be connected toone or more of the rods 66 rather than to the screw 20. In suchembodiments, a current supplied by the electrical power source 42 willbe supplied to the rods 66 which in turn will conduct the current to oneor more screws 20 integrally attached via the rods 66 (as discussed). Insuch embodiments, an electric current may be distributed evenly overeach screw 20 thus providing for uniform stimulation and/or powerconsumption by each screw.

According to some aspects, a current supplied to the screw 20 by thepower source 42 may be pulsed and/or may be provided intermittently. Forexample, in some embodiments the power source 42 may pulse directcurrent to the screw 20 (either directly or via the rod 66, etc., asdiscussed) following a predetermined time interval schedule. In otherembodiments, the power source 42 may provide direct current to the screw20 following a predetermined duty cycle. For example, for a 10% dutycycle, the power source 42 may supply electric current to the screw 2010% of the time while not supplying an electric current for 90% of thetime. In still other embodiments, the power source 42 may supply directcurrent to the screw 20 following a predetermined waveform or the like.For example, in some embodiments the power source 42 may supply a directcurrent to the screw by varying the amplitude of current being suppliedaccording to, e.g., a square wave, sine wave, etc. In any event,supplying a current to the screw 20 intermittently may provide benefitsover known power delivery systems, such as reduced electrochemicalreactions at the screw 20 surface, and improved tissue healing and boneformation. For example, applying a current to the screw 20 may stimulatean area of interest and thus provide the osteogenic benefits asdiscussed. However, intermittent periods of a reduced current or nocurrent being supplied may allow for periods of recovery and thus mayfurther promote bone growth, etc. Further, and particularly whencombined with a selectively applied anodization layer as discussed,embodiments of the invention may lead to reduced power consumptionand/or increased battery life. Reduced power consumption and/orincreased battery life may improve device longevity and reduce the needfor surgical replacement of the battery, thereby reducing clinical riskand complications for patients.

In some embodiments, a duty cycle/pulsed schedule, etc., applied to thescrew 20 may be varied according to a present phase of recovery for thebone and/or its surrounding tissue. For example, in some applications itmay be more beneficial to apply increased stimulation (e.g., electriccurrent) to an area of interest early in a recovery process. Thus, for aperiod of time directly following implanting the screw 20, a directcurrent may continuously or nearly continuously be provided to the screw20. However, as time passes and the area of interest begins to heal(e.g., new bone forms, etc.) the periods of stimulation may be reduced.For example, the duty cycle of an applied direct current may begradually reduced over time allowing for intermittent periods ofrecovery until the bone/tissue, etc., is fully healed. In one example, adirect current may be supplied to the screw 20 following a 100% dutycycle shortly after implanting the screw in the bone, such that the areaof interest is always stimulated early in the healing process. However,as the bone and/or its surrounding tissue begins to heal, the duty cyclemay be reduced such that the area of interest is provided both periodsof stimulation (i.e., periods when the current is supplied to the screw20) and periods of recovery (i.e., periods when the current is notsupplied to the screw). Accordingly, in some embodiments a combinationof a selectively applied anodization layer combined with an appropriateduty cycle may provide increased osteogenic benefits over knownstimulation techniques.

FIG. 10 is a simplified block diagram of a system 1000 according to thepresent disclosure. The system 1000 includes a power source 1002, acontroller 1004, a first electrode 1006, and a second electrode 1008.The system 1000 may be used for any suitable application describedherein. In some embodiments, the system 1000 is used for one or more ofstimulating bone growth, tissue healing, and pain control. The powersource 1002 provides power to the system 1000. In an example embodiment,the power source is a DC power source, such as one or more batteries,capacitors, photovoltaic modules, power converters (AC/DC or DC/DC),etc. In some embodiments, the power source 1002 includes the powersource 42.

The power source 1002 is coupled to controller 1004. The controller 1004controls and adjusts the application of DC current over time asdescribed herein. The controller 1004 may be any combination of digitaland/or analog circuitry suitable for controlling the application of DCcurrent as described herein. In some embodiments, the controller 1004includes a processor and a memory (not shown). The processor executesinstructions that may be stored in the memory. The processor may be aset of one or more processors or may include multiple processor cores,depending on the particular implementation. Further, the processor maybe implemented using one or more heterogeneous processor systems inwhich a main processor is present with secondary processors on a singlechip. In another implementation, the processor may be a homogeneousprocessor system containing multiple processors of the same type. Thememory is any tangible piece of hardware that is capable of storinginformation either on a temporary basis and/or a permanent basis. Thememory may be, for example, without limitation, random access memory(RAM) such as dynamic RAM (DRAM) or static RAM (SRAM), read-only memory(ROM), erasable programmable read-only memory (EPROM), electricallyerasable programmable read-only memory (EEPROM), non-volatile RAM(NVRAM), and/or any other suitable volatile or non-volatile storagedevice.

The controller is coupled to the first electrode 1006 and the secondelectrode 1008 to selectively direct current from the power source 1002to an area of interest 1010. The area of interest 1010 may be an area ora volume of a patient in which treatment using system 100 is desired. Inthe exemplary embodiment, the first electrode is a screw, such as thescrew 20. Although a single first electrode 1006 is illustrated, thesystem 1000 may include any number of first electrodes 1006, each ofwhich may selectively receive current from the controller 1004. Thesecond electrode 1008 is a ground electrode. The system 1000 may includeone or a plurality of ground electrodes 1008. The controller 1004controls application of current from the power source 1002 to the firstelectrode 1006. The current passes from the first electrode 1006,through the area of interest 1010, and to the second electrode 1008. Inthe illustrated embodiment, the second electrode is coupled to thecontroller 1004. In other embodiments, the second electrode is coupledto the power source 1002.

Although the components of the system 1000 are illustrated as separatecomponents, they may be separate components or may be integratedtogether. For example, in some embodiments, the controller is integratedwith the first electrode 1006. In some embodiments, the power source1002 is integrated with one or more of the controller 1004, the firstelectrode, and the second electrode. Moreover, the components of thesystem 1000 may be coupled together by any suitable wired or wirelessconnection. For example, the power source may inductively couple powerto the controller 1004 for distribution through the electrodes 1006 and1008.

The systems and methods described herein, including system 1000, may beused for many different clinical applications. For example, the systemsand methods described herein can be utilized in spinal surgery for thepurposes of accelerating bony fusion. They may be used in the design ofpedicle screws, lateral mass screws, cortical screws used in and aroundthe spine. They may also be used in the creation of custom spinalsystems and instrumentation including rods, plates, screw caps, tulips,clips, etc. The methods and systems may also be applied to screws and/orimplantable device(s) used in the creation and implementation ofinter-body spacers, artificial discs, and the like.

The systems and methods described herein can be utilized in the designof instrumentation for use in the case of a bony fracture, whichrequires internal fixation and the use of screws, instrumentation,and/or metallic hardware. They may be used in the design of corticalscrews used to stabilize and fix bony fractures of any bone. Morespecifically, the systems and methods may be used in the design ofcortical screws used to stabilize and fix bony fractures of long boneswith a high rate of non-union. The systems and methods may be used inthe design of pins, wires, rods, and/or plates used to fix and stabilizebroken, damaged, or diseased bone or bone tissues.

The systems and methods described herein can be applied to the design ofmetallic implants commonly used in joint reconstruction. For example,they may be used in the design of artificial metallic hip implantsinclude hip stems, femoral stems, femoral implants, acetabular implants,cups, and associated screws or metallic fixtures or instrumentation. Themethods and systems may also be used in the design of artificialmetallic knee joints, elbow joints, shoulder joints, etc. includingballs, stems, cups, and associated metallic fixtures andinstrumentation.

The systems and methods described herein may be utilized in dentalimplant systems including endodontic, orthodontic, and oral surgeryapplications. Specifically, they may be utilized in dental implantsystems such as dental posts, mandibular implants, screws, abutments,bridges, crowns, etc.

The systems and methods described herein may be utilized ininstrumentation and fixation devices for reconstructive surgery. Forexample, the systems and methods may be utilized in fixation systemsused to secure, mend, and fix broken bones in the face, hand, skull,etc. Moreover, they may be used to design screws, plates, and/orfixation systems for use in closing and fixating the skull followingneurosurgery, trauma, cranial closure, etc.

The example methods and systems may be used with metallic implants andscrews designed to resorb bone in areas of undue bone formation as aresult of pathologies or disease. For example, they may be utilized inpins or screws utilized to resorb bone and/or osteophytes surroundingjoints affected by osteoarthritis, rheumatoid arthritis, etc. They maybe used in the treatment of medical conditions involving globaloveractive or improper bone growth such as fibrodysplasia ossificansprogressive (FOP), diffuse idiopathic skeletal hyperostosis (DISH),ankylosing spondylitis, heterotopic ossification. Some embodiments maybe used for removal of bone masses in medical conditions involvingneoplastic bone formation or bony tumors such as osteosarcoma,chondrosarcoma, Ewing's sarcoma, osteoblastoma, osteoid osteoma, etc.Similarly, the methods and systems described herein may be used in theremoval of osteophytes (i.e. “bone spurs”) formed in the foot, shoulder,neck, spine, etc. as a result of chronic osteoarthritis, rheumatoidarthritis, reactive arthritis, rotator cuff injuries, plantar faciitis,spondylosis, and/or spinal stenosis.

Although various embodiments were described herein with reference tohuman applications, the methods and systems described herein may also beused in similar manners and for similar purposes in non-humanapplications, such as veterinary applications, including canine andequine applications.

When introducing elements of the present invention or the preferredembodiment(s) thereof, the articles “a”, “an”, “the” and “said” areintended to mean that there are one or more of the elements. The terms“comprising”, “including” and “having” are intended to be inclusive andmean that there may be additional elements other than the listedelements.

As various changes could be made in the above constructions withoutdeparting from the scope of the invention, it is intended that allmatter contained in the above description or shown in the accompanyingdrawings shall be interpreted as illustrative and not in a limitingsense.

What is claimed is:
 1. A screw for use in stimulating at least one ofbone growth, tissue healing, and pain control comprising: an elongateshaft having a length extending between opposite ends, an exteriorsurface, and a screw thread formed on the exterior surface of the shaftand extending along at least a portion of the length of the shaft, saidexterior surface of the shaft including at least a portion of the screwthread having an insulating coating extending along at least a portionof the length of the shaft; a head adjacent one end of the shaft forengaging the screw to rotate the screw and thereby drive it into bone;and an electrical conductor electrically connectable to the shaft forconveying current through the shaft, wherein a thickness of theinsulating coating at a first portion of the shaft is greater than athickness of the insulating coating at a second portion of the shaft. 2.The screw set forth in claim 1, wherein the insulating coating isdisposed adjacent to both the opposite ends of the shaft, and whereinthe insulating coating does not cover a middle portion of the shaftdisposed between the two opposite ends.
 3. The screw set forth in claim2, wherein the first portion is disposed at a first of the opposite endsof the shaft, wherein the second portion is disposed adjacent the middleportion of the shaft, and wherein the insulating coating decreases inthickness along the length of the shaft from the first portion to thesecond portion.
 4. The screw set forth in claim 1, wherein theinsulating coating extends between forty percent of the length of theshaft and ninety five percent of the length of the shaft.
 5. Anapparatus for stimulating at least one of bone growth, tissue healing,and pain control comprising: an electrical power source; and a pluralityof electrodes electrically connected to the electrical power source, atleast one of said electrodes has a tip and threads adapted for screwinginto a patient and an insulating coating extending along at least aportion of a length of the exterior surface of at least one electrodeand covering at least a portion of the threads, wherein a thickness ofthe insulating coating at a first portion of the at least one electrodeis greater than a thickness of the insulating coating at a secondportion of the at least one electrode.
 6. The apparatus set forth inclaim 5, wherein the insulating coating extends between forty percent ofthe length of the at least one electrode and ninety five percent of thelength of the at least one electrode.